Heat pump apparatus for regulating motor vehicle temperature

ABSTRACT

This temperature regulation apparatus comprises a heat pump ( 12 ) comprising a main compression circuit ( 14 ) for a refrigerant fluid taking heat from a cold source ( 16 ) and transferring it to a hot source ( 18 ). The cold source ( 16 ) comprises a first heat exchanger ( 24 ) for exchanging heat between the refrigerant fluid and a coolant liquid thermally coupling the main refrigerant circuit ( 14 ) to a first secondary coolant circuit ( 26 ) capable of being selectively connected to an “outside” heat exchanger ( 30 ) and to a “cold” heat exchanger ( 32 ). The hot source ( 18 ) comprises a second heat exchanger ( 34 ) for exchanging heat between the refrigerant fluid and a coolant liquid thermally coupling the main refrigerant circuit ( 14 ) to a second secondary coolant circuit ( 36 ) capable of being selectively connected to at least the outside heat exchanger ( 30 ) and to a “hot” heat exchanger ( 39 ). The invention is applicable in particular to air conditioning a motor vehicle cabin and/or to regulating the temperature of a part of said vehicle.

The present invention relates to improved heat pump apparatus forregulating motor vehicle temperature.

It applies in particular to air conditioning a motor vehicle cabinand/or to regulating the temperature of a part of said vehicle.

The state of the art, and in particular FR-2 697 210, discloses motorvehicle temperature regulation apparatus of the type comprising:

-   -   a heat pump comprising a main compression circuit for a        refrigerant fluid taking heat from a cold source to transfer it        to a hot source;    -   the cold source comprising a first heat exchanger for exchanging        heat between the refrigerant fluid and a coolant liquid,        thermally coupling the main refrigerant circuit to a first        secondary coolant circuit capable of being selectively connected        to at least two heat exchangers; and    -   the hot source comprising a second heat exchanger for exchanging        heat between the refrigerant fluid and a coolant liquid,        thermally coupling the main refrigerant circuit to a second        secondary coolant circuit capable of being selectively connected        to at least two heat exchangers.

As a general rule, the compression circuit comprises an evaporator inheat exchange with the cold source, and a condenser in heat exchangewith the hot source, these elements being connected together by acompressor and an expander. The refrigerant vaporizes in the evaporator,thereby taking heat from the cold source. The compressor draws in thevaporized refrigerant and delivers it into the compressor which iscooled by heat exchange with the hot source and in which the refrigerantcondenses. The condenser allows the refrigerant in the liquid state topass to the evaporator while lowering its pressure.

A heat pump can be used either to heat a space or a part or else to coolsaid space or said part.

In FR-2 697 210, the secondary circuits are capable of being selectivelyconnected via appropriate valves to an outside heat exchanger placed inthe engine compartment of the vehicle, and to an inside heat exchangerplaced in the vehicle cabin. By acting on the valves, it is possible touse the heat exchanger either to cool the vehicle cabin (in which casethe hot source is the outside heat exchanger and the cold source is theinside heat exchanger), or else to heat the cabin (in which case the hotsource is the inside heat exchanger and the cold source is the outsideheat exchanger).

It should be observed that in the second case the performance of theheat pump is limited by the temperature of the outside air. In the coldseason, air temperature is not sufficient to obtain satisfactoryefficiency for the heat pump when it is used for the purpose of heatingthe cabin. Furthermore, frosting of the cold source in the cold seasoncan degrade the performance of the heat pump in operation.

Furthermore, the reversibility of the outside and inside heat exchangers(each of which is used either as a hot source or as a cold source) leadsessentially to the following two drawbacks:

1) the performance of each heat exchanger is limited by the fact that itcannot be optimized for specific operation of the heat exchanger eitheras a hot source or else as a cold source; and

2) it is difficult to control the humidity of the air in the vicinity ofthe inside heat exchanger.

The second drawback is particularly troublesome when air conditioningthe vehicle cabin in mid-season. Under such circumstances, the insideheat exchanger is likely to be subjected to frequent changeovers in itsmode of operation between refrigerating and heating the cabin. Thus, inrefrigerating mode, the inside heat exchanger picks up water. Onswitching to heating mode, the water accumulated while refrigerating isexhausted into the cabin by evaporation. FR-2 697 210 proposesmitigating that drawback by means of an auxiliary heat exchanger placedclose to the inside heat exchanger. Nevertheless, that auxiliary heatexchanger is not functional under certain circumstances, in particularfor mid-season temperatures of around 15° C.

An object of the invention is to optimize the performance of heat pumptemperature regulation apparatus, in particular when it is used forheating or refrigerating the cabin of a motor vehicle, or indeed when itis used for regulating the temperature of a part of said vehicle.

For this purpose, the invention provides motor vehicle temperatureregulation apparatus of the above-specified type, characterized in thatthe first secondary circuit is capable of being selectively connected toan “outside” heat exchanger and to a “cold” heat exchanger, and in thatthe second secondary circuit is capable of being selectively connectedto the outside heat exchanger and to a “hot” heat exchanger.

According to other characteristics of various embodiments of thisapparatus:

-   -   the cold and hot heat exchangers are arranged in a cabin of the        vehicle;    -   the outside heat exchanger is a coolant/air heat exchanger;    -   the cold heat exchanger is a coolant/air heat exchanger;    -   the hot heat exchanger is a coolant/air heat exchanger;    -   the first secondary coolant circuit includes a three-port valve        connected to the outlet of the first refrigerant/coolant heat        exchanger of the cold source, to the inlet of the outside heat        exchanger, and to the inlet of the cold heat exchanger;    -   the second secondary coolant circuit includes a three-port valve        connected to the outlet of the second refrigerant/coolant heat        exchanger of the hot source, to the inlet of the outside heat        exchanger, and to the inlet of the hot heat exchanger;    -   the first and second secondary coolant circuits include a common        three-port valve connected to the inlet of the first        refrigerant/coolant heat exchanger of the cold source, to the        inlet of the second refrigerant/coolant heat exchanger of the        hot source, and to the outlet of the outside heat exchanger;    -   the first secondary circuit is capable of being selectively        connected to a supplementary outside heat exchanger;    -   the supplementary outside heat exchanger exchanges heat between        the coolant liquid and a vector of heat coming directly or        indirectly from a gas, in particular an exhaust gas of the        vehicle engine, a liquid, in particular a cooling liquid of the        vehicle engine, or a solid, in particular at least one        electrical or electronic component of the vehicle, said heat        vector being selected so that the supplementary outside heat        exchanger acts relative to the coolant circulating in the first        secondary circuit as a cold source that is hotter than the        outside heat exchanger;    -   the first secondary coolant circuit includes a supplementary        three-port valve connected to the outlet of the first        refrigerant/coolant heat exchanger of the cold source, to the        inlet of the outside heat exchanger, and to the inlet of the        supplementary outside heat exchanger;    -   the hot heat exchanger is connected to a branch of a cooling        liquid circuit of an engine of the vehicle, the downstream end        of the branch being connected to a coolant inlet of the hot heat        exchanger, and the upstream end of the branch being connected to        a coolant outlet of the hot heat exchanger;    -   the temperature regulation apparatus includes a subsidiary heat        exchanger in heat exchange with a part of the vehicle that is to        be temperature-regulated, the subsidiary heat exchanger being        connected in parallel with the cold and hot heat exchangers;    -   the first secondary coolant circuit includes a subsidiary        three-port valve connected to the outlet of the first        refrigerant/coolant heat exchanger of the cold source, to the        inlet of the subsidiary heat exchanger, and to the inlet of the        cold heat exchanger;    -   the second secondary coolant circuit includes a subsidiary        three-port valve connected to the outlet of the second        refrigerant/coolant heat exchanger of the hot source, to the        inlet of the subsidiary heat exchanger, and to the inlet of the        hot heat exchanger;    -   the first and second secondary coolant circuits include a common        subsidiary three-port valve connected to the inlet of the first        refrigerant/coolant heat exchanger of the cold source, to the        inlet of the second refrigerant/coolant heat exchanger of the        hot source, and to the outlet of the subsidiary heat exchanger;    -   the main refrigerant circuit having the two heat exchangers in        which the refrigerant circulates is constituted by a plurality        of segments grouped together in a single module in which the        refrigerant fluid circulates, said module being designed to be        connected to the secondary coolant circuits via the        refrigerant/coolant heat exchangers, said segments being        connected to one another preferably by welding; and    -   the coolant liquid in at least one of the first and second        secondary circuits is a mixture of water and antifreeze.

The invention will be better understood on reading the followingdescription given purely by way of example and made with reference tothe accompanying drawings, in which:

FIGS. 1 to 3 are diagrammatic views in three respective differentconfigurations of use of motor vehicle temperature regulation apparatusin a first embodiment of the invention;

FIGS. 4 to 6 are views similar to the preceding figures showing motorvehicle temperature regulation apparatus in respectively second, third,and fourth embodiments of the invention; and

FIG. 7 is a diagrammatic view of a heat pump using carbon dioxide as therefrigerant fluid and suitable for fitting to temperature regulationapparatus in any of the embodiments.

FIGS. 1 to 3 show motor vehicle temperature regulation apparatusconstituting a first embodiment of the invention, and given overallreference 10.

In the description below, two parts are said to be thermally coupledtogether when they exchange heat between each other by means of asuitable heat exchanger.

The temperature regulation apparatus 10 comprises a heat pump 12 havinga main refrigerant circuit 14 of the compression type taking heat from acold source 16 and transferring at least some of it to a hot source 18.

The cold and hot sources 16 and 18 are connected together by acompressor 20 (electrically or mechanically driven) and an expandervalve 22. The refrigerant vaporizes taking heat from the cold source 16.the compressor 20 draws in the vaporized refrigerant and delivers it tothe hot source where it condenses and cools. The expander valve 22allows the refrigerant to pass in liquid form towards the cold source 16by lowering its pressure. The refrigerant circulates around the circuit14 in the direction marked by arrows in FIG. 1.

The cold source 16 comprises a first refrigerant/coolant heat exchanger24 thermally coupling the main refrigerant circuit 14 to a firstsecondary circuit 26 for liquid coolant. This circuit includes a pump 28for circulating the liquid coolant, which pump is connected to the inletof the first heat exchanger 24.

The first secondary circuit 26 is capable of being selectively connectedto an “outside” heat exchange 30 and to a “cold” heat exchanger 32.

In the example shown in FIGS. 1 to 3, the outside heat exchanger 30 is acoolant/air heat exchanger placed in the engine compartment M of thevehicle, and the cold heat exchanger 32 is a coolant/air heat exchangerplaced in the vehicle cabin H.

The hot source 18 has a second refrigerant/coolant heat exchanger 34thermally coupling the main refrigerant circuit 14 to a second secondarycircuit 36 for liquid coolant. This second secondary circuit comprises apump 38 for circulating the liquid coolant, which pump is connected tothe inlet of the second heat exchanger 34.

The second secondary circuit 36 is capable of being selectivelyconnected to the outside heat exchanger 30 and to a “hot” heat exchanger39.

In the example shown in FIGS. 1 to 3, the hot heat exchanger 39 is acoolant/air heat exchanger placed in the vehicle cabin H.

A conventional fan (not shown in the figures) serves to cause a flow ofair to circulate through the cold and hot heat exchangers 32 and 39.

The heat pump 12 connected to the first and second heat exchangers 24and 34 is located, for example, in the engine compartment of thevehicle.

The refrigerant fluid circulating in the main circuit is of conventionaltype. This refrigerant fluid is selected, for example, from chlorine orfluorine derivatives of methane or ethane (Freon), hydrocarbons,ammonia, or carbon dioxide.

When using carbon dioxide, the heat pump 12 is fitted with conventionalparts that are specific to this use. FIG. 7 shows a heat pump 12 adaptedto using carbon dioxide. In this case, the circuit 14 includes anaccumulator A connected to the outlet of the first heat exchanger 24(also referred to as an “evaporator”) and an oil separator S (forseparating-out oil coming from the compressor 20) connected between thecompressor 20 and the second heat exchanger 34 (also referred to as the“gas cooler”). The carbon dioxide circuit 14 also has an internal heatexchanger I thermally coupling a branch of the circuit 14 connecting thesecond heat exchanger 34 to the expander valve 22 with a branch of thecircuit 14 connecting the accumulator A to the compressor 20. Theaccumulator A, the separator S, and the internal heat exchanger I areconventional parts commonly used in heat pumps making use of carbondioxide as the refrigerant fluid.

The coolant liquid circulating in the first or second coolant circuit 26or 36 is preferably a mixture of water and antifreeze (glycol).

The two secondary coolant circuits 26 and 36 are connected respectivelyto the outside, cold, and hot heat exchangers 30, 32, and 39 by means ofa three-port valve 40 in the first secondary circuit, a three-port valve42 in the second secondary circuit, and a three-port valve 44 common toboth secondary circuits.

The three-port valve 40 of the first secondary circuit 26 is connectedto the outlet of the first refrigerant/coolant heat exchanger 24 of thecold source, to the inlet of the-outside heat exchanger 30, and to theinlet of the cold heat exchanger 32.

The valve 42 of the second secondary circuit 36 is connected to theoutlet of the second refrigerant/coolant heat exchanger 34 of the hotsource, to the inlet of the outside heat exchanger 30, and to the inletof the hot heat exchanger 39.

The valve 44 common to both the first and the second secondary circuits26 and 36 for the coolant liquid is connected to the inlet of the firstrefrigerant/coolant heat exchanger 24 of the cold source, to the inletof the second refrigerant/coolant heat exchanger 34 of the hot source,and to the outlet of the outside heat exchanger 30.

The valves 40 to 44 are controlled by conventional electrical means.

FIG. 1 shows the configuration of the temperature regulation apparatus10 in vehicle cabin refrigerating mode. Under these circumstances, thevalves 40 to 44 are set so as to connect the first secondary coolantcircuit 26 to the cold heat exchanger and the second secondary coolantcircuit 36 to the outside heat exchanger 30.

The coolant of the first secondary circuit 26 takes heat from the cabinvia the cold heat exchanger 32 and delivers it to the vaporizedrefrigerant via the first refrigerant/coolant heat exchanger 24.

The coolant of the second secondary circuit 36 takes heat from thecondensed refrigerant via the second refrigerant/coolant heat exchanger34 and delivers it to outside the vehicle via the outside heat exchanger30.

FIG. 2 shows the apparatus 10 in a cabin-heating configuration. Underthese circumstances, the valves 40 to 44 are set so as to connect thefirst secondary coolant circuit 26 to the outside heat exchanger 30 andthe second secondary coolant circuit 36 to the hot heat exchanger 39.

The coolant of the first second secondary circuit 36 takes heat fromoutside the vehicle via the outside heat exchanger 30 and delivers it tothe vaporized refrigerant via the first refrigerant/coolant heatexchanger 24.

The coolant of the second secondary circuit 36 takes heat from thecondensed refrigerant via the second refrigerant/coolant heat exchanger34 and delivers it to the cabin via the hot heat exchanger 39.

FIG. 3 shows the apparatus 10 in a configuration in which both secondarycircuits 26 and 36 are connected simultaneously to the cold and hot heatexchangers 32 and 39 by appropriate settings of the valves 40 to 44.This configuration makes it possible, for example, to accelerate cabinheating while controlling the relative humidity therein.

FIGS. 4 to 6 show second, third, and fourth embodiments of thetemperature regulation apparatus 10 of the invention. In these FIGS. 4to 6, elements analogous to those in the preceding figures arcdesignated by identical references.

In the second embodiment of the invention shown in FIG. 4, the firstsecondary circuit 36 is capable of being selectively connected to thecold heat exchanger 32, to the outside heat exchanger 32, and to asupplementary outside heat exchanger 30A. This heat exchanger exchangesheat between the liquid coolant and a vector for heat coming directly orindirectly from a gas, in particular the exhaust gas of the vehicleengine, a liquid, in particular the cooling liquid of the vehicleengine, or a solid, in particular at least one electrical or electroniccomponent of the vehicle. This heat vector is selected so that thesupplementary outside heat exchanger 30A acts relative to the coolantcirculating in the first secondary circuit 26 as a cold source that ishotter than the outside heat exchanger 30.

The first secondary coolant circuit 26 has an additional three-portvalve 46 connected to the outlet of the first refrigerant/coolant heatexchanger 24 of the cold source 16, to the inlet of the outside heatexchanger 30, and to the inlet of the supplementary outside heatexchanger 30A.

This additional valve 46 enables the first secondary circuit 26 to beselectively connected to the outside heat exchanger 30 and to thesupplementary outside heat exchanger 30A. Thus, in the cold season, whenthe apparatus 10 is in its heating configuration, it is preferable toconnect the first secondary circuit 26 to the supplementary outside heatexchanger 30A instead of connecting it to the outside heat exchanger 30so as to avoid frosting it. Relative to the coolant circulating in thefirst secondary circuit 26, the supplementary outside heat exchanger 30Aforms a cold source that is hotter than the outside heat exchanger 30.

It should be observed that the supplementary outside heat exchanger 30Aadvantageously makes it possible to recover the heat dissipated byvarious parts of the vehicle, which heat would generally be lost.

In the third embodiment of the invention shown in FIG. 5, the heatexchanger 39 is suitable for restoring to the cabin heat taken from thecooling liquid of the vehicle engine.

For this purpose, the hot heat exchanger 39 is connected to a branch 48of a cooling liquid circuit for the vehicle engine 50. This engine isnaturally disposed in the engine compartment M. The downstream end ofthe branch 48 is connected to a cooling liquid inlet of the hot heatexchanger 39. The upstream end of the branch 48 is connected to acooling liquid outlet of the hot heat exchanger 39. Where appropriate, avalve 52 serves to control the flow rate of cooling liquid passingthrough the hot heat exchanger 39 from the upstream end of the branch48.

The third embodiment of the invention enables the cabin to be heated bymeans of heat taken from the engine cooling circuit by using the heatexchanger 39 without having recourse to an additional heat exchangerspecific to the cooling circuit.

The temperature regulation apparatus of the invention can be used forregulating temperature in a space of the vehicle other than the cabin,or for regulating the temperature of any part of the vehicle.

This possibility constitutes the subject matter of the fourth embodimentof the invention shown in FIG. 6.

In this case, the temperature regulation apparatus 10 has a subsidiaryheat exchanger 54 in heat exchange with a part of the vehicle whosetemperature is to be regulated, for example a part disposed inside thecabin such as an electronics unit. This subsidiary heat exchanger 54 isconnected in parallel with the cold and hot heat exchangers 32 and 39using means that are described below.

The first secondary coolant circuit 26 has a subsidiary three-port valve56 connected to the outlet of the first refrigerant/coolant heatexchanger 24 of the cold source, to the inlet of the subsidiary heatexchanger 54, and to the inlet of the cold heat exchanger 32.

The second secondary coolant circuit 36 has a subsidiary three-portvalve 58 connected to the outlet of the second refrigerant/coolant heatexchange 34 of the hot source, to the inlet of the subsidiary heatexchanger 54, and to the inlet of the hot heat exchanger 39.

The first and second secondary coolant circuits 26 and 36 have a commonsubsidiary three-port valve 60 connected to the inlet of the firstrefrigerant/coolant heat exchanger 24 of the cold source, to the inletof the second refrigerant/coolant heat exchanger 34 of the hot source,and to the outlet of the subsidiary heat exchanger 54.

The settings of the three-port valves 56 to 60 enable the temperature ofthe part whose temperature is to be regulated to be controlled bycontrolling the flow rates of the liquid coolant in the secondarycircuits 26 and 36 and passing through the subsidiary heat exchanger 54.

Amongst the advantages of the invention, it should be observed that thecold and hot heat exchangers 32 and 39 have functions that are specific,one being for cooling and the other for heating. As a result theiroperation can be optimized.

Furthermore, the main refrigerant circuit 14, comprising in particularthe two heat exchangers 24 and 34 having the refrigerant flowingtherethrough, the compressor 20, and the expander valve 22, isconstituted by a plurality of segments that are advantageously groupedtogether in a single module within which the refrigerant circulates. Thesegments are preferably interconnected by welding so as to formconnections that are strong and leaktight and capable of withstandinghigh pressures, e.g. pressures exceeding 100 bars (10⁷ Pa), inparticular when the refrigerant is constituted by carbon dioxide.

The module constituted by the refrigerant circuit may be assembled andfilled with refrigerant prior to being mounted in the vehicle. On beingmounted in the vehicle, the module is connected to the secondary coolantcircuits 26 and 36 via the refrigerant/coolant heat exchangers 24 and34.

The module, which is advantageously mounted in the vehicle enginecompartment, is isolated from the cabin, thus avoiding any risk ofrefrigerant leaking into the cabin, and protecting the cabin from noisenuisance.

1. A motor vehicle temperature regulation apparatus, comprising: a heat pump comprising a main compression circuit for a refrigerant fluid taking heat from a cold source to transfer it to a hot source; the cold source comprising a first heat exchanger for exchanging heat between the refrigerant fluid and a coolant liquid, thermally coupling the main refrigerant circuit to a first secondary coolant circuit, capable of being selectively connected to an outside heat exchanger and to a cold heat exchanger; the hot source comprising a second heat exchanger for exchanging heat between the refrigerant fluid and a coolant liquid, thermally coupling the main refrigerant circuit to a second secondary coolant circuit, capable of being selectively connected to the outside heat exchanger and to a hot heat exchanger; and a subsidiary heat exchanger in heat exchange with a part of the vehicle that is to be temperature-regulated, the subsidiary heat exchanger being connected in parallel with the cold and hot heat exchangers.
 2. The apparatus according to claim 1, wherein the cold and hot heat exchangers are arranged in a cabin of the vehicle.
 3. The apparatus according to claim 1, wherein the outside heat exchanger is a coolant/air heat exchanger.
 4. The apparatus according to claim 1, wherein the cold heat exchanger is a coolant/air heat exchanger.
 5. The apparatus according to claim 1, wherein the hot heat exchanger is a coolant/air heat exchanger.
 6. The apparatus according to claim 1, wherein the first secondary coolant circuit includes a three-port valve connected to an outlet of the first heat exchanger of the cold source, to an inlet of the outside heat exchanger, and to an inlet of the cold heat exchanger.
 7. The apparatus according to claim 1, wherein the second secondary coolant circuit includes a three-port valve connected to an outlet of the second heat exchanger of the hot source, to an inlet of the outside heat exchanger, and to an inlet of the hot heat exchanger.
 8. The apparatus according to claim 1, wherein the first and second secondary coolant circuits include a common three-port valve connected to an inlet of the first heat exchanger of the cold source, to an inlet of the second heat exchanger of the hot source, and to an outlet of the outside heat exchanger.
 9. The apparatus according to claim 1, wherein the first secondary circuit is capable of being selectively connected to a supplementary outside heat exchanger and the supplementary outside heat exchanger exchanges heat between the coolant liquid and a vector of heat coming directly or indirectly from a gas, in particular an exhaust gas of the vehicle engine, a liquid, in particular a cooling liquid of the vehicle engine, or a solid, in particular at least one electrical or electronic component of the vehicle, said heat vector being selected so that the supplementary outside heat exchanger acts relative to the coolant liquid circulating in the first secondary circuit as a cold source that is hotter than the outside heat exchanger.
 10. The apparatus according to claim 9, wherein the first secondary coolant circuit includes a supplementary three-port valve connected to an outlet of the first heat exchanger of the cold source, to an inlet of the outside heat exchanger, and to an inlet of the supplementary outside heat exchanger.
 11. The apparatus according to claim 1, wherein the hot heat exchanger is connected to a branch of a cooling liquid circuit of an engine of the vehicle, a downstream end of the branch being connected to a coolant inlet of the hot heat exchanger, and an upstream end of the branch being connected to a coolant outlet of the hot heat exchanger.
 12. The apparatus according to claim 1, wherein the first secondary coolant circuit includes a subsidiary three-port valve connected to an outlet of the first heat exchanger of the cold source, to an inlet of the subsidiary heat exchanger, and to an inlet of the cold heat exchanger.
 13. The apparatus according to claim 1, wherein the second secondary coolant circuit includes a subsidiary three-port valve connected to an outlet of the second heat exchanger of the hot source, to an inlet of the subsidiary heat exchanger, and to an inlet of the hot heat exchanger.
 14. The apparatus according to claim 1, wherein the first and second secondary coolant circuits include a common subsidiary three-port valve connected to an inlet of the first heat exchanger of the cold source, to an inlet of the second heat exchanger of the hot source, and to an outlet of the subsidiary heat exchanger.
 15. The apparatus according to claim 1, wherein the main refrigerant circuit having the first and second heat exchangers in which the refrigerant fluid circulates is constituted by a plurality of segments grouped together in a single module in which the refrigerant fluid circulates, said module being designed to be connected to the secondary coolant circuits via the first and second heat exchangers, said segments being connected to one another preferably by welding.
 16. The apparatus according to claim 1, wherein a coolant liquid in at least one of the first and second secondary circuits is a mixture of water and antifreeze. 